1. Field of the Invention
This invention relates to radio location systems for determining the identity and location of a plurality of locator-transmitter devices at arbitrary fixed or moving locations throughout a geographical location. My invention finds application in public safety systems, commercial trucking and taxi dispatching operations, personal alarm systems, and the like. This invention is related in part to my invention entitled "Ultra-Narrow Band Communication System", U.S. Pat. No. 4,117,405.
2. Prior Art
Systems for locating vehicles and/or persons within a city or extended area, sometimes referred to as automatic vehicle monitoring (AVM) systems, have long held the interest of both public safety authorities and commercial enterprises for the control of vehicles, for dispatching operations, and for citizens' alarm devices. Indeed, many different types of systems have been proposed in the past and some are currently undergoing testing. These systems utilize such radio methods as LORAN, OMEGA, radio pulse tri-lateration, and similar wireless techniques. Another radio location method employs what are referred to as "sign-posts" which are in fact either miniature radio transmitters or receivers located at various street intersections and within city blocks. These latter devices are intended to communicate with passing vehicles and thereby determine their position subsequent to which this information is communicated to a central location within the city. The latter systems suffer the problem that a considerable number of such sign-posts are necessary for accurate coverage within a city; these numbers ranging upwards of 10,000 or more sign-posts, and these pose difficult maintenance and installation problems as well as very high cost. On the other hand the wireless or radio technique like LORAN are often inaccurate, have complex and costly communication devices and receivers are necessary on each vehicle. Furthermore, none of these prior art systems are readily adaptable to low cost miniature packaging such as might be necessary for on-person alarm application (herein referred to as citizens' alarm).
Another prior art radio location system which was designed by the present inventor utilizes three existing local radio broadcast stations phase-locked to each other; that is, one broadcast station serves as the "master" station and the other two derive their RF carrier by phase-locking to the master station and thus all three stations are "synchronized." A receiver within a vehicle is designed to tune to and "track" all three broadcast stations and make phase comparisons in a manner designed to disclose the position of the vehicle, relative to some starting reference location.